Can you please do show me how to install joomla on /var/www/ (root) folder
I don't want /var/www/joomla and I think I am stuck with how to create permission and too afraid to make writable to public.
Can you do step by step please....

Well, download your joomla version from here : http://www.joomla.org/download.html
then go to the terminal and do sudo unzip pathtojoomla.zip -d /var/www
this will create a folder in the /var/www try extract files and directories from this folder and put them directly into /var/www (use sudo nautilus /var/www to access the /var/www using the nautilus browser)
for the permissions use 755 for directories and 644 for the files use chmod to set permissions.